1. Field of the Invention
The present invention pertains to cryostats used to produce cryogenic refrigeration by expansion of a working fluid (e.g. argon, nitrogen, carbon dioxide) through a Joule-Thompson Orifice. The cryostat can be placed inside of a dewar or other receptacle so that an inventory of liquefied working fluid can be maintained to cool an object such as an infrared detector. Cryostats according to the present invention are of the combined demand flow and fixed flow type which includes means to control the flow of working fluid through the orifice in response to temperature changes in the working fluid.
2. The Prior Art
Demand flow cryostats have been used in cryo-electronic systems such as for cooling infrared detectors and the like. Systems employing this type of detector can be used in ground operation and in airborne detection systems.
Demand flow cryostats of the type wherein flow control is achieved by sensing the presence or absence of a liquefied gas at the cold end of the heat exchanger and using the sensing device to control the size of the Joule-Thompson orifice is shown in U.S. Pat. No. 3,517,525. In these devices operation is normally in an on-off mode because the sensing mechanism is in contact with the liquefied working fluid so that before the sensor will react it must be warmed above the temperature of the liquid at the top of the insulating dewar within which such cryostats are mounted. A significant improvement over the abovementioned cryostats is disclosed in U.S. Pat. No 3,728,868, the spcification of which is incorporated herein by reference.
In addition, to the above other demand flow cryostats wherein an attempt to eliminate thermal cycling are shown in U.S. Pat. Nos. 3,747,365, 3,704,597, and 3,818,720.
British Pat. No. 1,238,470 discloses a demand flow cryostat wherein a bellows actuated needle valve is actuated by varying the pressure on the bellows disposed inside the mandrel. The cryostat includes a sensor below the valve which is used to signal an external valve between the mandrel and a source of fluid under pressure.
U.S. Pat. No. 3,827,252 discloses a dual orifice cryostat wherein a minimum flow is maintained by the fixed orifice and the variable orifice is utilized continuously to control the rate of refrigeration above the minimum value.